METHODS Preparation and Characterization of the Gold Hydrosols. Small GNPs with a diameter of ∼5.0 nm were prepared by the following procedure: To 90 mL of deionized water (“Millipore”), 1 mL of 1 wt % HAuCl4‚3H2O was added, and the mixture was stirred for 1 min; then, 2 mL of 38.8 mM sodium citrate was added, and the reaction mixture was stirred for another minute. A volume of 1 mL of fresh 0.075 wt % NaBH4 in 38.8 mM sodium citrate was added, and the reaction mixture was stirred for 5 min.